1. Field of the Invention
This invention relates to a process and apparatus for the thermal treatment of gypsum (CaSO.sub.4.2H.sub.2 O) by indirect heating to produce a hemihydrate product (CaSO.sub.4.O.sub.2 5H.sub.2 O).
2. Description of the Prior Art
Generally, it is known that in order to obtain a plaster which is rich in hemihydrate and is suitable for prefabrication, the method of calcination must first be carried out at a high temperature which imparts the required reactivity to the product; secondly, the gypsum must be homogeneous to as to avoid the presence of unburned material (non-dehydrated gypsum) and also of overburned material (or anhydrite II), sometimes caused by prolonged local contact with gases having a temperature and a partial pressure of water vapor which are incompatible with the stability of the hemihydrate form. Finally, the treatment must be economical and preferably continuous.
Many processes are known which are partially effective in achieving the desired results mentioned above. In the processes of German Applications Nos. P 17 71502 and P 17 58566 the reaction rate of the product undergoing reaction is achieved by mechanical movement of a part of the calcination apparatus, in rotating furnaces or in mobile hearth furnaces with direct or indirect heating. In all cases, the low coefficient of heat exchange requires large sizes of apparatus and hence high investment costs; likewise, it is also necessary to use dust elimination systems; in addition, the moving parts require maintenance which is not insignificant expense.
The reacting product is conveyed by means of a gas in the so-called "flash" calcination and pneumatic transport processes, as described in French Patent Application No. 2,202,251 and German Patent Applications Nos. P 22 00,532 and P 21 52,940. These processes are flexible but have the disadvantage of requiring large volumes of air, which, in the absence of complex recycling, prevents any possibility of controlling the partial pressure of water vapor of the system undergoing conversion. In contrast, the air/plaster ratio is much lower in the process of French Application No. 74/22,621 of the applicants, due to an apparatus of a special geometric design which makes it possible to operate at extremely high temperatures and to achieve a uniform conversion coupled with an excellent thermal efficiency.
Vertical transport of the material and calcination in a fluidized bed are also known, through French Patent No. 1,338,126 and its Addition No. 87,866, but until now a high, gypsum-fed fluidized bed required a high rate of fluidization, causing substantial dispersions and necessitating high-capacity blowers; moreover, such a device suffers from the drawbacks of using a single homogeneous reactor, in that the feeding of the reactant, at the outlet, reflects a compromise between the quality of the product and the size of the apparatus. Furthermore, the partial pressure of steam can only be controlled with difficulty in these processes which have not undergone industrial development.
The horizontal apparatus, in the form of a tunnel, of French Pat. No. 1,288,836, where the material is agitated and advances by virtue of intermittent pulses of steam is also known. The steam/air ratio is high, which has the advantage of allowing a choice of temperature and the steam pressure. However, the heat exchange between the material and the heating elements or walls is only intermittent, limiting the maximum productivity of the apparatus. In addition, the pulsation apparatus is complex and costly.